(1) Field of the Invention
The present invention relates to a developing device using one-component toner, more specifically, relates to a developing device used for an image forming device such as copying machines using electrophotography, printers and electrostatic recording devices.
(2) Description of the Related Art
The developing device using one-component toner is easy to reduce the size and easy to handle, but there is a problem in transferring and charging the toner and making the toner uniformly thin. A general developing device will now be described with reference to FIG. 1.
As shown in FIG. 1, an electrostatic image is formed on the surface of a photosensitive drum 51, which is an electrostatic latent image carrier, by means of a latent image-forming means (not shown). A developing roller 100 is disposed in a position opposite to the photosensitive drum 51 as a developer carrier, and on the surface of the developing roller 100, toner 2 which is the developer is adhered. The development is carried out by adhering the toner 2 to the photosensitive drum 51 to visualize the electrostatic image. The charged toner 2 is formed in a thin layer on the developing roller 100.
As for the means for forming toner in a thin layer, the toner 2 is first supplied and adhered on the developing roller 100 by a toner feed roller 200 which is a developer feed member brought into contact with the developing roller 100 and is connected to a bias power supply 210 for feeding toner. Thereafter, the toner 2 on the developing roller 100 is formed uniformly in a thin layer by a blade 300 which is a developer regulating member, as the developing roller 100 rotates. Furthermore, voltage for the developing bias is impressed to the developing roller 100 by a power supply 110 and blade bias voltage is impressed to the blade 300 by a power supply 390.
With such a developing device 1b using one-component developer, however, when the residual toner adhered to the developing roller 100 is not removed after the development, the developing pattern formed by the residual toner remains until the next development, resulting in a memory image. To solve this problem, a Mylar sheet 430 (a residual toner removing member) abutting to the developing roller 100 after the development is newly provided to scratch the residual toner on the developing roller 100.
With the developing device using conventional non-magnetic developer (hereinafter referred to as "toner"), as a method for supplying the toner, there have been disclosed many methods, such as the one using an elastic foamed body which is disclosed in Japanese Unexamined Patent Publication No. Hei 2-191974. The one using the elastic foamed body as the method for supplying the toner is capable of supplying the toner to the developer carrier (hereinafter referred to as a "developing roller"), but it has been difficult to remove the residual toner on the developing roller which passed through the developing zone. In particular, when the supply voltage having the same polarity as the charging polarity of the toner is impressed in order to supply toner sufficiently and stably, removal of the residual toner on the developing roller becomes more difficult. As a result, in the toner layer on the developing roller, toner which was not consumed in the developing zone and toner newly supplied by the developing roller mingle together to deteriorate the developing properties and cause a development history (development unevenness).
Therefore, as disclosed in Japanese Patent Publication No. Sho 60-7790, there is a method in which a roller for recovering the residual toner from the developing roller is provided in a prescribed location, and bias voltage having the opposite polarity to the charged polarity of the toner is impressed on the recovering roller to remove the residual toner on the developing roller after it passed through the developing zone. With this method, however, a roller for recovering the toner is separately required, and the cost increases.
Moreover, as disclosed in Japanese Patent No. 2557826 (issued in 1996), there is a method in which a thin plate having elasticity is brought into contact with the developing roller to remove the residual toner on the developing roller. Moreover, there is another method in which a thin plate is brought into contact with the developing roller and bias voltage is impressed to the thin plate to remove the residual toner on the developing roller.
With these methods, when the residual toner on the developing roller is removed, it is necessary to bring the toner removing member into deep contact with the developing roller. Furthermore, in the case of removing the residual toner on the developing roller electrically, the discharging effect is increased by pressing the toner removing member into deep contact with the developing roller and enlarging the contact area.
As a result, a mechanical stress is applied to the residual toner on the developing roller, and the toner is easily deteriorated, thus deterioration of the developing properties, such as filming of the toner on the surface of the developing roller may be caused.
On the other hand, with the technique disclosed in Japanese Unexamined Patent Publication No. Hei 2-191974, it is described therein that the cell density of the elastic foamed body is preferably from 10 cells per 25 mm to 200 cells per 25 mm, and cells are brought into contact with each other so that the contact depth is from 0.5 mm to 2 mm and the contact width is from 0.2 mm to 5 mm, or cells are closely arranged so that the distance between the both surfaces is not larger than 2 mm. However, the above-mentioned cell density corresponds to the cell density of almost all foamed elastic body, and there may be a problem occurred at the time of actual use. For example, if the cell density is low, the toner easily penetrates into the foamed cell of the elastic foamed body to make the elastic foamed body hard. As a result, increase of the driving torque of the feed roller or decrease of the volume of toner transferred by the feed roller is caused. On the contrary, if the cell density is high, the toner cannot easily penetrate into the foamed cell of the elastic foamed body, but the volume of toner transferred by the feed roller decreases. As a result, the density necessary for development cannot be obtained, and since the hardness of the elastic foamed body itself is high, the torque increases, the contact pressure between the developing roller and the feed roller becomes high, and the toner is subjected to the mechanical stress and becomes easily deteriorated.
Furthermore, since the contact conditions has a wide range, the feed properties differ largely. For example, if the contact depth is large, the contact pressure between the developing roller and the feed roller becomes high, the driving torque of both rollers increases, and the toner is subjected to the mechanical stress and easily deteriorated. On the contrary, if the contact depth is small, the capability of the feed roller to apply the toner to the developing roller decreases, and the feed roller cannot feed the toner sufficiently. In addition, the accuracy of the outer diameter of the feed roller is required, but the preparation thereof with the elastic foamed body is difficult.
Furthermore, with a feeding method using an elastic foamed body as a feed member, the developing roller and the feed member are brought into contact with each other. In this case, the developing roller having hardness higher than that of the feed roller is normally used. For example, as a material of the developing roller, a metal sleeve or an elastic solid rubber is often used. Therefore, in the contact area of the developing roller and the feed roller, modified feed rollers using an elastic foamed body having hardness lower than that of the developing roller accounts for the majority thereof, and the state of the contact pressure between the developing roller and the feed roller is mostly determined by the hardness of the feed roller. That is to say, even if the contact depth of the developing roller and the feed roller is the same amount, if the hardness of the feed roller is high, the pressure in the contact area of the developing roller and the feed roller increases, on the contrary, if the hardness of the feed roller is low, the pressure in the contact area decreases.
Furthermore, with a toner feeding method using an elastic foamed body, toner adhered on the surface of the feed roller and toner infiltrating shallowly into the inside of cells are transferred to the surface of the developing roller, as the feed roller rotates. The toner which has reached the contact area of the developing roller and the feed roller is frictionally charged and moves to the surface of the developing roller. At that time, however, the toner transferred to the surface of the developing roller is only the toner existing close to the outermost layer on the surface of the feed roller, and the quantity of toner mechanically applied to the developing roller decreases, hence poor supply of toner is easily caused on the developing roller. At the time of developing solid black, sufficient image density cannot be obtained.
Therefore, when the followability of the image density at the time of printing solid black is kept during development, if supply bias having the same polarity as the charging polarity of the toner is applied to the toner feed roller in order to feed toner sufficiently and stably, the effect of discharging member obtained by mechanically removing the residual toner on the developing roller with the feed roller is hardly obtained. That is to say, the electric adhesion of the residual toner with the developing roller after passing through the developing zone is decreased by the discharging member, but by applying the supply bias to the feed roller, the residual toner removed by the mechanical force of the feed roller adheres again on the surface of the developing roller due to the electric field in the feeding direction of the toner, formed between the developing roller and the feed roller, in the vicinity of disjunction or in the contact area of the feed roller and the developing roller. As a result, there is a possibility to cause a difference in the charge quantity or a difference in the adhesion quantity per unit area between the residual toner on the developing roller and the newly supplied toner, and it appears as density unevenness (development history) on the output image.